Prostate cancer (PCa), the second leading cause of cancer-related death in US men, presents the greatest racial disparity of any malignant disease in African Americans, with a 1.6-1.9 times higher incidence rate and 2- 3 times higher mortality rate than Caucasians. PCa in African Americans presents with more advanced clinical stages and aggressive tumors. PCa biomarkers are needed to improve early detection of clinically significant tumors and to distinguish indolent or slow growing tumors from more aggressive ones. In our search for tumor markers, we cloned prosaposin (PSAP) as a secreted protein and discovered its overexpression in metastatic PCa cells and tissues. Previously, we demonstrated that PSAP and/or its active molecular derivatives increase migration and invasion and upregulate matrix-degrading proteolytic enzymes expression in PCa cells. In Caucasians, serum-PSAP levels are increased in metastatic PCa when compared to primary PCa or normal prostate tissues. In addition, there was no difference in serum-PSAP levels between organ-confined and locally-invasive tumors. In contrast to Caucasians, our pilot study in African Americans shows that a) tissue expression of PSAP is significantly higher in undifferentiated tumors with Gleason grade 4/5 pattern than in the differentiated tumors with Gleason grade d 3 or benign glands (BPH) and b) serum-PSAP levels are higher in locally invasive (stage III/IV) tumors than in the organ-confined (stage I/II) tumors which positively correlates with disease progression. Based on these observations, we hypothesized that PSAP contributes to PCa progression and has the characteristics of a novel biomarker discriminating the aggressive tumors from non- aggressive ones in African American patients. To test our hypothesis, we propose the following Aims: 1) Define the clinical significance of serum-PSAP as a marker of PCa progression or aggressiveness in African Americans; 2) Determine the association between tissue expression of PSAP and clinical and histopathological predictors or prognosticators of PCa progression or aggressiveness in African Americans; and 3) Determine the association between PSAP and invasive and metastatic phenotypes in PSAP-overexpressed or -silenced African American PCa cells. We will use immunohistochemical analysis and sandwich-ELISA assays to quantify PSAP expression levels in a large pool of tissue and serum samples and to determine their association with clinicohistopathological predictors or prognosticators of PCa aggressiveness and progression in African Americans. We will also test the effect of increased or decreased PSAP expression on tumor growth rate and spontaneous metastatic ability in subcutaneous and orthotopic tumor xenografts in African American- derived PCa cells. At the conclusion of this proposal, we will have defined the clinical and histopathological significance of PSAP as a biomarker of PCa progression and/or aggressiveness in African Americans that could be further utilized in clinical development.